Apparatus and method for detecting gaseous contaminants

ABSTRACT

Method of detecting gaseous contaminants by monitoring the rate of diffusion of a given gas through a selectively permeable metallic surface which is reversibly poisoned by the contaminants. Apparatus for practicing method.

tilted States Patent 91 [11 3,718,434

Pierce [451 Feb. 27, 1973 s41 APPARATUS AND METHOD FOR 3,437,446 4/1969Pierce ..23/254 E DETECTING GASEOUS 3,567,383 3/ 1971 Langley ..23/232 ECONTAMINANTS Inventor: Russell W. Pierce, Broadway & Elm

Streets, Hanover, Mass. 02339 Filed: Jan. 13, 1972 Appl. No.: M7583 US.Cl ..23/232 R, 23/254 R, 73/23 R References Cited UNITED STATES PATENTSMatle et a] ..23/254 X SAMPLE LINE FROM C3 H PROCESS FEED PrimaryExaminer-Morris 0. Wolk Assistant Examiner-R. E. Serwin Attorney-JosephM. Lane et al.

57 AiBSTRACT Method of detecting gaseous contaminants by monitoring therate of diffusion of a given gas through a selectively permeablemetallic surface which is reversibly poisoned by the contaminants.Apparatus for practicing method.

24 Claims, 1 Drawing Figure l0 [a 48 g 52 f m IS 50 as 5 4e vacuum] VENT22 24 Q A 44 2a r as B 4* DP H RECORDER, CELL X s s L PATENTEB FEBZ 7I975 SAMPLE LINE FROM 0 H PROCESS FEED IO v :48 5; I I

40 r 38 I4 LIZ l8 50 T? 4s VACUUM L52 22 24 VENT A 2o 32 28 1 B 4 .25.(T RECORDER-L CELL X X APPARATUS AND METHOD FOR DETECTING GASEOUSCONTAMINANTS BACKGROUND OF THE INVENTION The field of the presentinvention is a method and device for detecting the presence of gaseouscontaminants.

Since the man-generated gaseous sulphur compounds which are exhaustedinto the troposphere as pollutants, are represented principally by S S0and H 8, an estimation of the sum of these three chemicals is usuallysufficient to warn of dangerously high atmospheric sulphur levels. Thesecompounds, even in small concentrations, are poisonous to life. Thus,accurate detection of these pollutants is necessary to enableenforcement of aid pollution controls.

The present invention can be used to monitor the ambient air, stackgases, flue gas streams, automotive exhaust, etc. for the presence ofthe above-mentioned contaminants. This invention also relates to anapparatus and method for the protection of catalysts employed in variouschemical manufacturing processes.

Prior art methods for the detection of such contaminants, either in theatmosphere or in industrial gas streams, which involve analysiswithchemical reagents, are too cumbersome and too slow to be of muchpractical utility. Other known methods tend to be insensitive andexpensive to install and operate.

My earlier invention (US Pat. No. 3,437,446, issued Apr. 8, 1969)utilizes the reversible poisoning effect of sulphur compounds onpalladium as a means of detecting the presence of those sulphurcompounds. My earlier invention monitored the poisoning effect (andsulphur vcompound concentration) by measuring changes in the electricalresistivity of the palladium element.

SUMMARY OF THE INVENTION The present invention monitors the poisoningeffect (and contaminant concentration) by measuring changes in the flowrate for a gas diffusing through a selectively permeable metallicelement. The changes in the rate of diffusion occur as the metallicsensing element is successively poisonedby the contaminants and purgedby a diffusible gas.

Accordingly, it is an object of the present invention to provide animproved device for the detection of gaseous contaminants.

Another object is to provide a rapid, sensitive and relativelyinexpensive means for the detection of gaseous contaminants.

Yet another object of this invention is to provide an apparatus for amethod of detecting sulphur-containing compounds such as sulphur dioxidein gaseous environments.

Other objects and further scopeof applicability of the present inventionwould become apparent from the detailed description to follow, taken inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic flow diagramrepresent- DESCRIPTION OF THE PREFERRED EMBODIMENTS At the outset, theinvention is described in its broadest aspects.

The present invention involves a means for detecting a gaseouscontaminant which is capable of reversibly poisoning a sensing element.This invention monitors the poisoning effect (and contaminantconcentration) by measuring the rate of diffusion of a diffusible gasthrough a selectively permeable metallic element. The metallic sensingelement is fonned with external and internal diffusing surfaces. Therate of diffusion undergoes relatively large changes as the permeablemetal element is successively poisoned by the contaminant and purged bythe diffusible gas. The changes in the rate of diffusion are largeenough to be easily measured by standard instruments such asdifferential pressure cells. The sensing element may be a metallic foil,metallic tube, or a supported metallic surface that is permeable tohydrogen or other diffusible gas. The method of detection involvesbringing a standard gas stream containing a constant concentration ofthe diffusible gas into contact with the external diffusing surface ofthe permeable metal portion of the sensing element and maintaining alower concentration of the diffusible gas at the internal diffusingsurface thereby creating a partial pressure differential for thediffusible gas across the permeable metal and causing that gas todiffuse therethrough. The lower concentration for the diffusible gas atthe internal diffusing surface may be maintained by purging that surfaceby a vacuum technique, or by any other suitable method. A sample of thegaseous environment to be tested is mixed with the gas stream cominginto contact with the external diffusing surface of the sensing element.If the sample contains a contaminant, the sensing element will becomepoisoned and thereby less permeable. The resumption of flow of anon-contaminated gas stream containing a diffusible gas serves to purgethe metallic sensing element and thereby restore its permeability. Theflow of diffusible gas through the element may be measured directly(e.g., monitoring pressure changes in the diffusing gas stream on theinternal side of the sensing element) or indirectly (e.g., monitoringthe pressure changes in the gas stream passing across the externaldiffusing surface of the sensing element).

In the preferred embodiment, the sensing element is made from an alloycontaining approximately 60 percent palladium and 40 percent copper. Thehydrogen permeability of this alloy is several times as great as that ofpalladium. Accordingly, the sensitivity of the device of this inventionis greatly increased when the sensing element is made from the abovealloy.

An even higher degree of permeability can be obtained by constructingthe metallic sensing element in the form of a metallic layer on a poroussubstrate.

Palladium metal and some of its alloys are remarkably permeable togaseous hydrogen under certain circumstances. The driving force whichcauses a gas, e.g. hydrogen, to diffuse through sections of those metalsis a concentration gradient arising from a hydrogen partial pressuredifferential across the metal foil or membrane. As the AP is increased,the rate and volume of gas diffusion through the metal also increases.The hydrogen diffusion rate through a square centimeter of purepalladium foil, one one-thousandth of an inch thick and held at 350C, isapproximately one-tenth of a standard liter per minute per atmosphere ofpressure difference across the palladium foil. A delta AP can beestablished across the sensing element by evacuation of the downstreamatmosphere using any standard vacuum apparatus or by means of purgingthe downstream atmosphere with another gas, e.g. nitrogen.

In the case of hydrogen diffusion through palladium, diatomic hydrogenis first adsorbed onto the external palladium surface in an amountdependent on the upstream hydrogen pressure. Dissociation of thehydrogen then occurs and the hydrogen atoms enter the interior latticeof the metal to form something analogous to a metal-hydrogen alloy. Thisalloy, while showing appreciable strength associated with its Pd-Hbonds, is not spatially fixed and individual hydrogen atoms move aboutrather freely subject to a variety of influences, including their ownconcentration gradient and the temperature of the metal. In thediffusion process, the hydrogen atoms eventually reach the downstream orinternal metallic surface where the reverse of the upstream surfacephenomenon occurs, and the atomic hydrogen reacts to form diatomicmolecules which then desorb as a gas.

It appears that in the case of metal of thicknesses less than oneone-thousandth of an inch thick, that the overall diffusion rate islimited primarily by the external and internal surface adsorption anddesorption rates. In contrast, in the case of foils thicker than threeor four thousandths of an inch, the overall diffusion rate is limited bythe transport rates for the movement of hydrogen atoms (protons) throughthe interior lattice of the metal.

The poisoning effect on the palladium metal produced by sulfur compoundsinvolves the chemisorption of sulfur atoms at the surface of thepalladium metal. ln this manner, sites otherwise available for hydrogenadsorption become strongly bonded to the relatively large sulfur atomsand thus sterically block the incoming hydrogen molecules from contactwith the palladium. This poisoning effect is caused by both the largesize of the adsorbed sulfur atom and by the high bond strength of thesulfur-palladium bond. The addition of 1 ppm H 8 to the hydrogencontaining feed stream will reduce the hydrogen diffusion rateapproximately l percent. The hydrogen diffusion rate will remainconstant so long as the H S:H ratio in the feed stream remainsunchanged. Surprisingly, the hydrogen diffusion rate will return to itsinitial value if the H 8 is removed from the feed stream.

The combined effect of increasing P 'QTYE, is inversely proportional tothe hydrogen sulphide partial pressure and directly proportional to thehydrogen partial pressure. By P is meant the equivalent of any sulfurcompound which can react with H over palladium at 350C to form H 8. Arate equation for the combination of the two effects may be written asfollows:

H n.eir m/was where;

t= diffusion rate of hydrogen through the metal foil. Expressed in std.cm /cm atm/mil of foil thickness,

P P partial pressures of hydrogen sulphide and hydrogen, respectively,in atmospheres, and

k a proportionality constant having appropriate units and havingdifferent values which increase with the operating temperature.

If we operate the apparatus at some constant hydrogen partial pressuresuch as one-half atmosphere, for example, then eq.l becomes:

H- B) A which is true, at least for short ranges of P1125 at constantEven though the proportionality and linearity implied in eq.2 is by nomeans true over wide ranges of hydrogen sulphide to hydrogen partialpressure ratios, the hydrogen diffusion rate behavior with respect tohydrogen sulphide levels is reproduceable so that one is able tocalculate hydrogen sulphide levels accurately from calibration data.

It has been discovered that the flow rates for hydrogen, both throughand across the sensing element, undergo significant and easily measuredchanges in accordance with the changes in the P T ratio discussed above.

The present invention involves various methods of dynamic flowmeasurement as means for monitoring the changes in the rate of hydrogendiffusion which in turn correspond to changes in the amounts of sulfurcontaminants in the feed gas. The dynamic flow measurement may be madeby means of differential pressure apparatus or other conventionalapparatus.

The sensing element used in the detection system of this invention is aselectively permeable metallic element. By selective it is meant thatthe metal of the sensing element is permeable with respect to only aspecific diffusible gas, e.g. hydrogen, but is impervious to other gasesin its environment. The physical shape and structure of the permeablemetallic element may assume any one of a wide variety of differentforms. The sensing element may be a thin foil, capilliary tube, or athin film of the desired metal deposited on a microporous substrate,e.g. a porous ceramic, porous vycor glass, or a nickel-electric-arcsmoke-plated, sintered composite.

For the detection of sulfur compounds the metal employed in the sensingelement may be palladium, platinum, palladium alloys such aspalladium-silver, palladium-copper, palladium-nickel, andpalladiumcopper-nickel alloys, or other metals selectively permeable tohydrogen and capable of being reversibly poisoned by the contaminantsbeing monitored. An alloy containing approximately 40 weight per centcopper, and the balance palladium has been found to be particularlyeffective when used for the purposes of this invention. The 40 percentcopper 60 percent palladium alloy has a hydrogen permeability severaltimes as great as that of palladium alone. Accordingly, detectorsutilizing this alloy are much more sensitive and have a quicker responsetime.

Substrates which may be used to support a metallic film, to form asensing element, may have microporous structures, i.e. interconnectedpassageways of holes on the order of 10 to Angstrom units (1A 10 cm) indiameter. The metal film may be deposited on the substrate by anyconventional method such as by vapor plating, electroplating, vacuumdeposition and the like. Sensing elements of this type, wherein thesupported metallic film has a thickness of 0.5 to mil, are preferredbecause the surface to volume ratio between the metallic film surfaceand the volume of the gas stream to be monitored is maximized, therebyminimizing the bulk diffusion rate limitation on the response time ofthe sensing element.

Vapor deposition techniques can be used to produce the supportedpalladium sensing element. In one such process, palladiumacetylacetonate is vaporized in a stream of an inert gas such as drynitrogen or argon at 190-2l0C and brought into contact with a suitablesubstrate maintained at 220-250C to decompose the salt and leave acoating of palladium on the substrate. instead of nitrogen or argon,carbon dioxide, methane, or another suitable gas may be used as thecarrier. Diallyl di-chloro palladium or another volatile palladium saltmay be substituted for the palladium diacetylacetonate.

Another method which has been successfully employed to produce supportedpalladium sensing ele ments is the so-called smoke arcplating process.This process utilizes a relatively coarse base support material, forexample, a 40 mesh porous stainless steel filter. The base material iscoated with nickel smoke which is produced by an electric are betweentwo nickel electrodes. Leaving the high temperature area of the electricarc plasma, the vaporized nickel condenses in the form of sphericaldrops on the surface of the stainless steel filter. The smoke depositionis continued until a layer of nickel dust from five to ten thousandthsinch has been formed. The composite is then placed in a hydrogenatmosphere and sintered at approximately 500C for several minutes.Thesintered composite is compacted between polished metal mandrels undera pressure of several tons per square inch. Highly polished, hardenedmandrel faces are essential, especially opposing the side of thesubstrate covered by the nickel arc smoke. Sheets of freshly cleavedmica may be placed between the mandrel and the smoke coated surface ofthe composite. The smoke deposition, sintering, and compaction steps arerepeated until the layers of compacted and sintered nickel depositionstotal several thousandths of an inch. The composite is then vapor platedin a conventional manner to form a palladium coated sensing element.

In an alternative embodiment, changes in the pressure differentialacross a metallic diaphragm are measured by monitoring the deflection ofthe diaphragm. The apparatus used in this embodiment is an electronicpressure meter of the type marketed by MKS Instruments, Inc., under theTrademark BARATRON and which has been modified by replacing thediaphragm metal with a palladium, or copper-palladium foil and by makingprovision for the heating of the palladium diaphragm. If the downstreamside of the palladium diaphragm is evacuted and the exit from theupstream side restricted, the diaphragm of the above apparatus will bedeflected sufficiently so that it will be sensitive to a poisoningeffect which would serve to temporarily decrease the amount ofdiffusion, increase the pressure differential, and thereby increase thedeflection.

Operation of the apparatus of this invention may be best understood byreference to the drawing. For purposes of illustration, a system formonitoring a propylene process is shown. A detector, generallydesignated by the numeral 10, is shown provided with a palladium foil112, a gas inlet 14, and gas outlets 16 and 110. Pure hydrogen from abottle enters the system through line 22, which flow is controlled byvalve 24. Similarly, pure propylene is admitted into the system from asource 26 through line 28 and is controlled by valve 30. The purehydrogen and propylene mix at 32 and are directed through feed line 34to the detector inlet 14 located at the upstream side of the palladiumfoil 12. An injection valve 36 is located in line 34 which valveperiodically injects a sample of the propylene process stream, from line38, into feed line 34. If the injected sample contains a sulfurcontaminant, the Pd foil 12 will be poisoned to an extent dependent uponthe sulfur compound to hydrogen ratio of the injected sample. When sucha poisoning occurs, the amount of hydrogen leaving the detector 10through outlet 16 will decrease and the amount of hydrogen leavingthrough outlet 18 will increase. This alteration of the hydrogen flowproduces higher pressure at the upstream or internal side of the foil inspace 40, and a lower pressure at the downstream or external side inspace 42. In this example a differential pressure cell 44 -is connectedto the upstream space 40 by line 46 to monitor the pressure in thatspace which pressure is continuously recorded (the recorder is notshown). Changes in the recorded pressure are converted into values forthe concentration of contaminants in the injected samples. Thisconversion of pressure change values into contaminant concentrationvalues may be performed manually by reference to a calibration curvebased on known values for standard samples. The conversion of values canalso be automatic if an integrator-recorder is used.

Alternately, the line 46 can be connected to the downstream space 42 sothat the pressure there can be monitored.

In practice, SOcc/min each of pure H and pure C H flow into the cell andout exits 16 and 18 through lines 48 and 50, respectively, to the vacuumapparatus 52. A vent 54 is provided in line 50 as a means of controllingthe pressure differential between spaces 40 and 42 and therebycontrolling the rate of diffusion.

The foil 12 is heated, by any conventional means to avoid the phenomenonknown as hydrogen embrittlement." Heating the foil also serves toincrease the hydrogen diffusion rate and, therefore, the sensitivity ofthe apparatus. A suitable operating temperature is approximately 350C.The heated palladium or other foil must not be allowed to cool below acritical temperature while exposed to hydrogen. The foil will swell,embrittle, or crack, or in any event develop leaks, if the hydrogen isnot flushed or otherwise removed before allowing the foil to cool belowthe critical temperature. This effect is believed to be caused byhydrogen protons occupying a position in the lattice between thepalladium atoms causing a spreading of the lattice greater than thatwhich can be accommodated by the elasticity of the metal below thecritical temperature. The critical temperature varies with the hydrogenpartial pressure, the lower the H partial pressure, the lower thecritical temperature. Hydrogen in Metals, D.

P. Smith (University of Chicago Press, 1948), at page 86, gives thecritical temperature as 310C at hydrogen partial pressures of oneatmosphere. Experiments conducted with hydrogen partial pressures of twoatmospheres have determined the critical temperature to be approximately327C. It follows that hydrogen must never be admitted when starting theapparatus from room temperature and must be removed from the apparatusbefore cooling below the critical temperature.

Instead of the differential pressure apparatus 44, a Katharometer can beused. If the space 42 is swept with nitrogen, the thermal conductivityof the gaseous mixture in space 42 is raised because hydrogen is a muchbetter heat conductor than is nitrogen. Katharometry forhydrogen-nitrogen mixtures requires very' modest apparatus for highsensitivity to changes in the hydrogen-nitrogen ratio. The Katharometercan also be calibrated to give read-out values representative of variouslevels of sulfur contaminants in the process feed stream.

lf the concentration of sulfur contaminants in the process feed is low,and their presence intermediate, line 28, the propylene source 26, andthe injection valve 36 may be eliminated. In this manner the processfeed 38 can be continuously monitored without the sample injectionprocedure. As in the earlier described procedure, it is essential thatthe concentration of hydrogen entering at 14 remain constant.

In addition to its obvious utility in the field of environmentalprotection, this invention also relates to an apparatus and method forthe protection of catalysts employed in various chemical manufacturingprocesses. It has long been known that precious metal hydrogenationcatalysts (e.g., Pt, Pd, etc.) are very sensitive to such gases ashydrogen sulphide when these gases are present in the feed streams tothe catalysts. The poisoning effect can be of the order of half theusual catalytic activity, even when H 8 is present only to the extent ofl to 2 parts per million.

The various embodiments described above relate to the detection ofgaseous sulfur compounds. However, the invention may be applied to thedetection of any gaseous contaminant capable of reversibly poisoning aselectively permeable metallic element. For example, silver and certainsilver alloys are permeable to oxygen and are reversibly poisoned bynitrogen oxides. Accordingly, a detector employing silver or a silveralloy could be used in the manner described above to-monitor thepresence of nitrogen oxides in various gaseous environments.

The nickel and nickel alloy permeable elements previously described mayalso be used in combination with carbon monoxide, employed as thediffusible gas. The carbon monoxide permeation rates through thesenickel and nickel alloy elements is markedly affected by poisoning byHCN, NH, and amines. Although the transport rates for carbon monoxidethrough nickel are lower than those for hydrogen through nickel, thepresence of HCN, NH and amines, as contaminants, may be monitored by asensitive apparatus such as the modified BARATRON, previously described.

In yet another embodiment, the apparatus of the present invention,employing a palladium element, may be used to detect ozone. An importantindex of air pollution is the determination of atmospheric ozone inparts per billion (ppb) or less concentrations. It has been found thatwith the palladium element maintained at a constant level of sulfurpoisoning, the presence of a small amount of ozone will serve to purgethe element and thereby increase the rate of diffusion. These increasesin the rate of diffusion can be measured and interpreted as valves forthe concentration of ambient ozone by any of the methods previouslydescribed in connection with the detection of sulfur compounds.Experimentally, a high-voltage leak tester was connected to the externalleads of the diffuser cell. The tester was found to produce enough ozoneto momentarily clean off" the permeability reducing layer of PdS.Because Ozone is highly reactive, the major difficulty with itsdetection is the problem of preserving the 0 until it can be assayed.This problem may be overcome by introducing the sample suspected ofcontaining ozone into the apparatus separately at a point very close tothe external diffusing surface of the palladium or palladium alloyelement.

Other changes and modifications in the devices and procedures describedabove will become apparent to those skilled in this art and obvious andequivalent changes are intended to be included in the scope of thisinvention.

1 claim:

1. An apparatus for detecting the presence of a contaminant in a gaseousenvironment comprising:

1. a sensing element containing a metal permeable to a specificdiffusible gas, said metal being capable of being reversibly poisoned byadsorption and desorption of the contaminant and said metal havinginternal and external diffusing surfaces;

2. means for bringing a gas stream containing a constant concentrationof said diffusible gas into contact with said external diffusing surfaceof said metal;

3. means for removing said diffusible gas from said internal surface ofsaid metal to maintain a gaseous atmosphere at said internal surfacehaving a concentrationof said diffusible gas less than that of said gasstream, thereby creating a partial pressure differential for saiddiffusible gas to cause said gas to diffuse through said metal at agiven rate; means for mixing a sample of the gaseous environment to betested with said gas stream;

5. means for monitoring said rate of diffusion through said permeablemetal; and

6. means for purging said contaminant from said permeable metal.

2. The apparatus of claim 1, wherein said permeable metal is shaped intoa foil.

3. The apparatus of claim 1, wherein said permeable metal is selectedfrom a palladium and palladium alloys.

4. The apparatus of claim 1, wherein said permeable metal is selectedfrom silver and silver alloys.

5. The apparatus of claim 1, wherein said permeable metal is apalladium-copper alloy containing approximately 40 percent by weightcopper.

6. The apparatus of claim 1, wherein said permeable metal is in the formof a capillary tube.

7. The apparatus of claim 1, wherein said sensing element comprises acoating of said permeable metal on a porous glass substrate.

8. The apparatus of claim 1, wherein said sensing element comprises acoating of said permeable metal on a sintered metal substrate.

9. The apparatus of claim 1, wherein said means for monitoring is adifferential pressure cell.

10. The apparatus of claim 1, wherein said means for monitoring is aKatharometer.

11. The apparatus of claim 1, wherein said means for mixing is aninjection valve which periodically injects samples of the gaseousenvironment to be tested into said gas stream.

12. The apparatus of claim 1, wherein said means for mixing allows acontinuous flow of the gaseous environment to be tested to enter intosaid gas stream.

13. A process for detecting the presence of contaminants in a gaseousenvironment, which process employs a sensing element having a metalportion, which metal portion is permeable to a specific diffusible gasand is provided with internal and external diffusing surfaces and whichis capable of being reversibly poisoned by adsorption and desorption ofthe contaminants, said process comprising the following steps:

1. bringing a gas stream containing a constant concentration of thediffusible gas into contact with the external diffusing surface of themetal;

2. removing the diffusible gas from the internal diffusing surface ofthe metal to maintain a concentration of the diffusible gas at theinternal diffusing surface less than that in contact with the externaldiffusing surface, thereby creating a partial pressure differential forthe diffusible gas and causing the diffusible gas to diffuse through themetal at a given rate;

3. mixing a portion of the gaseous environment to be tested with saidgas stream;

. measuring any changes in said rate of diffusion of the diffusible gasthrough the permeable metal portion of the sensing element; and

5. purging any contaminants from the permeable metal by intermediatecontacting with a contaminant-free diffusible gas.

14. The process of claim 13, wherein said removal of the diffusible gasfrom the internal diffusing surface is effected by means of a vacuum.

15. The process of claim 13, wherein said removal of the diffusible gasfrom the internal diffusing surface is effected by means of purging withan inert gas. i

16. The process of claim 13, wherein changes in said rate of diffusionof the diffusible gas through the permeable metal portion of the sensingelement are measured by means of a differential pressure cell.

17. The process of claim 13, where changes in said rate of diffusion ofthe diffusing gas through the permeable metal portion of the sensingelement are measured by means of a Katharometer.

18. The process of claim 13, wherein the metal which is permeable to aspecific diffusible gas is selected from palladium and palladium alloysand wherein the diffusible gas is hydrogen.

19. The process of claim 13, wherein the metal which is permeable to aspecific diffusible gas is selected from palladium and palladium alloysand wherein the diffusible gas is carbon monoxide.

20. The process of claim 13 wherein said gas stream additionallycontains a constant concentration of a sulfur compound and wherein saidpurging is effected by ozone present in the portion of the gaseousenvironment tested.

21. The process of claim 13, wherein the metal which is permeable to adiffusible gas is selected from silver and silver alloys and wherein thediffusible gas is oxygen.

22. The process of claim 13, wherein said gas stream is continuouslymixed with a portion of the gaseous environment to be tested prior tocontact with the external diffusing surface of the permeable metalportion of the sensing element.

23. The process of claim 13, where samples of the gaseous environment tobe tested are periodically injected into said gas stream.

24. An apparatus for detecting the presence of a contaminant in agaseous environment comprising:

1. a sensing element in the form of a deflectable metal diaphragm, saidmetal being capable of being reversibly poisoned by adsorption anddesorption of the contaminant and said metal having internal andexternal diffusing surfaces;

. means for bringing a gas stream containing a constant concentration ofsaid diffusible gas into contact with said external diffusing surface of-said metal diaphragm;

3. means for establishing a pressure differential between the space atthe external diffusing surface and the space at the internal diffusingsurface, to cause a measurable deflection of said diaphragm;

. means for mixing a sample of the gaseous environment to be tested withsaid gas stream;

5. means for monitoring said deflections of said diaphragm; and

6. means for purging said contaminant from said permeable metal.

2. The apparatus of claim 1, wherein said permeable metal is shaped intoa foil.
 2. removing the diffusible gas from the internal diffusingsurface of the metal to maintain a concentration of the diffusible gasat the internal diffusing surface less than that in contact with theexternal diffusing surface, thereby creating a partial pressuredifferential for the diffusible gas and causing the diffusible gas todiffuse through the metal at a given rate;
 2. means for bringing a gasstream containing a constant concentration of said diffusible gas intocontact with said external diffusing surface of said metal;
 2. means forbringing a gas stream containing a constant concentration of saiddiffusible gas into contact with said external diffusing surface of saidmetal diaphragm;
 3. means for removing said diffusible gas from saidinternal surface of said metal to maintain a gaseous atmosphere at saidinternal surface having a concentration of said diffusible gas less thanthat of said gas stream, thereby creating a partial pressuredifferential for said diffusible gas to cause said gas to diffusethrough said metal at a given rate;
 3. means for establishing a pressuredifferential between the space at the external diffusing surface and thespace at the internal diffusing surface, to cause a measurabledeflection of said diaphragm;
 3. mixing a portion of the gaseousenvironment to be tested with said gas stream;
 3. The apparatus of claim1, wherein said permeable metal is selected from a palladium andpalladium alloys.
 4. The apparatus of claim 1, wherein said permeablemetal is selected from silver and silver alloys.
 4. measuring anychanges in said rate of diffusion of the diffusible gas through thepermeable metal portion of the sensing element; and
 4. means for mixinga sample of the gaseous environment to be tested with said gas stream;4. means for mixing a sample of the gaseous environment to be testedwith said gas stream;
 5. means for monitoring said rate of diffusionthrough said permeable metal; and
 5. means for monitoring saiddeflections of said diaphragm; and
 5. purging any contaminants from thepermeable metal by intermediate contacting with a contaminant-freediffusible gas.
 5. The apparatus of claim 1, wherein said permeablemetal is a palladium-copper alloy containing approximately 40 percent byweight copper.
 6. The apparatus of claim 1, wherein said permeable metalis in the form of a capillary tube.
 6. means for purging saidcontaminant from said permeable metal.
 6. means for purging saidcontaminant from said permeable metal.
 7. The apparatus of claim 1,wherein said sensing element comprises a coating of said permeable metalon a porous glass substrate.
 8. The apparatus of claim 1, wherein saidsensing element comprises a coating of said permeable metal on asintered metal substrate.
 9. The apparatus of claim 1, wherein saidmeans for monitoring is a differential pressure cell.
 10. The apparatusof claim 1, wherein said means for monitoring is a Katharometer.
 11. Theapparatus of claim 1, wherein said means for mixing is an injectionvalve which periodically injects samples of the gaseous environment tobe tested into said gas stream.
 12. The apparatus of claim 1, whereinsaid means for mixing allows a continuous flow of the gaseousenvironment to be tested to enter into said gas stream.
 13. A processfor detecting the presence of contaminants in a gaseous environment,which process employs a sensing element having a metal portion, whichmetal portion is permeable to a specific diffusible gas and is providedwith internal and external diffusing surfaces and which is capable ofbeing reversibly poisoned by adsorption and desorption of thecontaminants, said process comprising the following steps:
 14. Theprocess of claim 13, wherein said removal of the diffusible gas from theinternal diffusing surface is effected by means of a vacuum.
 15. Theprocess of claim 13, wherein said removal of the diffusible gas from theinternal diffusing surface is effected by means of purging with an inertgas.
 16. The process of claim 13, wherein changes in said rate ofdiffusion of the diffusible gas through the permeable metal portion ofthe sensing element are measured by means of a differential pressurecell.
 17. The process of claim 13, where changes in said rate ofdiffusion of the diffusing gas through the permeable metal portion ofthe sensing element are measured by means of a Katharometer.
 18. Theprocess of claim 13, wherein the metal which is permeable to a specificdiffusible gas is selected from palladium and palladium alloys andwherein the diffusible gas is hydrogen.
 19. The process of claim 13,wherein the metal which is permeable to a specific diffusible gas isselected from palladium and palladium alloys and wherein the diffusiblegas is carbon monoxide.
 20. The process of claim 13 wherein said gasstream additionally contains a constant concentration of a sulfurcompound and wherein said purging is effected by ozone present in theportion of the gaseous environment tested.
 21. The process of claim 13,wherein the metal which is permeable to a diffusible gas is selectedfrom silver and silver alloys and wherein the diffusible gas is oxygen.22. The process of claim 13, wherein said gas stream is continuouslymixed with a portion of the gaseous environment to be tested prior tocontact with the external diffusing surface of the permeable metalportion of the sensing element.
 23. The process of claim 13, wheresamples of the gaseous environment to be tested are periodicallyinjected into said gas stream.
 24. An apparatus for detecting thepresence of a contaminant in a gaseous environment comprising: